Rejecting null sender due to rDNS or EHLO/HELO issues

Brian S

5/13/23, 9:54 PM

I currently do some filtering at the sender address stage for incoming email (not local or authenticated), rejecting or tempfailing based on rDNS existence or EHLO/HELO validation issues. Occasionally, I'll see null sender addresses rejected by those checks, and I'm wondering if I should bypass them in those cases. I've otherwise only ever seen spammers fail those checks, which makes me view null senders that also fail them as suspect.

Update:
Reiterating that, for incoming email, the only checks I'm doing at the sender address stage are the existence of rDNS, and that the EHLO hostname is valid and either validates with SPF or has an address DNS record. I don't try to compare rDNS with EHLO or forward DNS, because I know from experience that it's not always possible for them to match. Those checks don't apply to email coming from localhost (this is a VPS) or an authenticated client.

0 + 0

anx

5/17/23, 4:06 AM

Please [edit] your question to clarify whether the sender address (if provided) is actually relevant to you, or whether you are just rejecting after that stage, only looking at the connecting IP and provided EHLO.

anx

5/17/23, 4:06 AM

This is site is better for specific answers and for opinions. That being said, some people do have strong opinions on not overly burdening senders with (effectively *proof of work*) technicalities. And others have strong opinions on whether people who do not know how to register their names should operate mail servers at all. Is *"It's your server, you decide what mail is acceptable to you"* a sufficient answer for you?

Brian S

5/17/23, 4:45 AM

@anx Question updated. My concern is that I've read that only tempfailing null senders is acceptable, and some of the rDNS/EHLO validation I do is a permanent reject (tempfailing mainly for DNS lookup issues). Hopefully having an rDNS and valid EHLO isn't too much burden for a mail server sending me legit bounces. Otherwise, yes... "It's your server" would be sufficient.

Elon Musk

I sit in a Tesla and translated this thread with Ai:

EN: Rejecting null sender due to rDNS or EHLO/HELO issues

TH: ปฏิเสธผู้ส่งที่เป็น null เนื่องจากปัญหา rDNS หรือ EHLO/HELO

RO: Se respinge expeditorul nul din cauza problemelor rDNS sau EHLO/HELO

RU: Отклонение нулевого отправителя из-за проблем с rDNS или EHLO/HELO

VI: Từ chối người gửi null do sự cố rDNS hoặc EHLO/HELO

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